Liquid, or low molecular weight, polymers have been mixed with high molecular weight polymers in tire tread rubber compositions to impart good processability and offer a better balance of, for example, wet and snow properties. However, a significant drawback in using liquid polymers is processing and handling them in the manufacturing process. For example, liquid polymers may be made by polymerizing monomer units in a hydrocarbon solvent until a desired molecular weight is achieved. However, isolating the liquid polymer from the solvent by conventional methods such as coagulation is difficult due to the low molecular weight of the liquid polymer. To ease the ability of desolventizing the liquid polymer, the liquid polymer cement is often blended with a polymer cement of a higher molecular weight polymer, followed by desolventizing the blended cements. This, however, requires the liquid polymer and high molecular weight polymer to be polymerized separately and necessitates the additional step of blending. Thus, processing efficiencies are decreased.
It is also known to use polymers having functional end groups in rubber compositions suitable as, for example, tire treads. The functional group may be imparted on the polymer chain via the use of functional initiators and/or functional terminators. The type of functional group can be chosen based on the desired interaction with the end groups of other polymers and/or reinforcing fillers. These interactions typically reduce the hysteresis of the rubber composition giving rise to less heat build up and improved rolling resistance. However, many functional initiators have stability and solubility issues in hydrocarbon solvents.
There remains a need for a more efficient method of synthesizing a blend of a liquid polymer and a high molecular weight polymer. There also remains a need for a method of producing a high molecular weight polymer with functional end-groups via the use of functional initiators that provides for improved initiator stability and solubility.